1. Field of the Invention
The present invention relates to a technology for suppressing torque ripple caused by an induced voltage distortion in a permanent magnet motor.
2. Description of Related Art
The following is a method known in the related art that may be adopted to suppress torque ripple when the waveform of a voltage induced in a motor is rectangular when there is a distortion in the induced voltage. In the method, three-phase induced voltage waveforms are converted to a d-axis voltage ed and a q-axis voltage eq and the torque ripple is canceled out based upon a q-axis current command value Iqref calculated as expressed in (1) below.
                              I          qref                =                              2            3                    ⁢                      (                                          T                ref                            ×                                                ω                  m                                                  e                  q                                                      )                                              (        1        )            
Tref and ωm in expression (1) respectively represent a torque command value and a mechanical angular speed at the motor. Namely, the q-axis current command value Iqref containing a pulsation waveform can be calculated as expressed in (1) so as to achieve uniformity in the motor torque even when the waveforms of the induced voltages at the motor are rectangular. The torque ripple can be suppressed by executing vector control in conformance to this current command value.
However, since the current command value Iqref contains the pulsation waveform, the q-axis current Iq corresponding to the current command value Iqref must be controlled by utilizing a high-response current control system. In other words, the current control response frequency must be set so as to achieve the relationship expressed in (2) below.n×p/2×ω≦upper limit to current control response frequency  (2)
n in expression (2) represents a wave component, the order of which is equal to or above 5% of the amplification component resulting from frequency analysis of an artificial rectangular wave simulated for the motor. In addition, P represents the number of poles of the motor and ω represents the actual rotation rate of the motor.
The torque ripple suppression method described above is disclosed in patent reference 1.    (Patent reference 1) Japanese Laid Open Patent Publication No. 2004-201487
Through the torque ripple suppression method in the related art described above, control under which the torque ripple component that occur when the waveforms of the induced voltages at the motor are rectangular is canceled out, is enabled. The motor torque in this situation is in proportion to the product of a q-axis induced voltage coefficient (notated as Ked), which is in proportion to the q-axis voltage eq, and the q-axis current Iq. For this reason, a torque ripple DC component is generated in correspondence to the product (ΔKed×ΔIq) of a pulsation component ΔKed in the q-axis induced voltage coefficient Ked and a pulsation component ΔIq in the current Iq. This torque ripple DC component, however, cannot be effectively utilized in the torque ripple suppression method in the related art.
An object of the present invention is to provide a torque ripple suppression control device and an electric power steering system that enable effective utilization of the torque ripple DC component.